Page 47 - Solar Power in Building Design The Engineer's Complete Design Resource

SOLAR POWER SYSTEM COMPONENTS 17

■ Overvoltage relay
■ Overfrequency relay
■ Underfrequency relay

Most inverters designed for photovoltaic applications are designed to allow simulta-
neous paralleling of multiple units. For instance, to support a 60-kW load, outputs of
three 20-kW inverters may be connected in parallel. Depending on the power system
requirements, inverters can produce single- or three-phase power at any required volt-
age or current capacity. Standard outputs available are single-phase 120 V ac and
three-phase 120/208 and 277/480 V ac. In some instances step-up transformers are
used to convert the output of 120/208 V ac inverters to higher voltages.

Input and output power distribution To protect inverters from stray spikes
resulting from lightning or high-energy spikes, dc inputs from PV arrays are protected
by fuses, housed at a junction box located in close proximity to the inverters.
Additionally, inverter dc input ports are protected by various types of semiconductor
devices that clip excessively high voltage spikes resulting from lightning activity.
To prevent damage resulting from voltage reversal, each positive (+) output lead with-
in a PV cell is connected to a rectifier, a unidirectional (forward-biased) element.
Alternating-current output power from inverters is connected to the loads by means of
electronic or magnetic-type circuit breakers. These serve to protect the unit from external
overcurrent and short circuits.

Grid-connected inverters In the preceding we described the general function of
inverters. Here we will review their interconnection to the grid, which requires a thor-
ough understanding of safety regulations that are mandated by various state agencies.
Essentially the goal of design safety standards for inverters used in grid-connected
systems, whether they be deployed in photovoltaic, wind turbine, fuel cell, or any
other type of power cogeneration system, is to have one unified set of guidelines and
standards for the entire country. Standard regulations for manufacturing inverters
address issues concerning performance characteristics and grid connectivity practices
and are recommended by a number of national test laboratories and regulatory
agencies.

Underwriters Laboratories For product safety, the industry in the United States
has worked with Underwriters Laboratories (UL) to develop UL1741, Standard for
Static Inverter and Charge Controller for Use in Independent Power Systems, which
has become the safety standard for inverters being used in the United States. Standard
UL1741 covers many aspects of inverter design including enclosures, printed circuit
board configuration, interconnectivity requirements such as the amount of direct
current the inverters can inject into the grid, total harmonic distortion (THD) of the
output current, inverter reaction to utility voltage spikes and variations, reset and
recovery from abnormal conditions, and reaction to islanding conditions when the util-
ity power is disconnected.